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Questions Related to Stress Analysis:- Part-2

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11. How do you calculate the stress developed due to thermal expansion?
Stress developed, Σ = ∈ x α , (α= Δ L/L)
……. Where, E = young’s modulus.
Δ L = increase in length due to thermal expansion.
L = original length of the pipe.
12. How do you calculate the thermal expansion in a pipe?
 Δ L = α x Δ T x L
In the codes and many reported calculations, α is used as inclusive of Δ T. Thus the above formula is
written as:- Δ L = α x L
Where, α = coefficient of thermal expansion from ambient to operating temperature .
L = length of the pipe.
13. What do you mean by Stress Intensity Factor (SIF)? Give some examples.
Stress Intensity Factor (SIF) is the ratio of maximum stress intensity to normal stress. It is used as safe factor to account for the effect of localised stress on piping under respective loading. In piping it is applied to welds, fittings, branch connections etc where stress concentration and possible fatigue failure may occur.
Example: – SIF for reducer and weldneck flange is 1.0
SIF for socket weld flange is 1.3

14. How much should be the pressure for hydro – test?
Hydrotest pressure should be ca lculated as follow except as provided against point No D.
A. 1.5 times of design pressure.
B. For a design temperature above the test temperature, minimum test pressure can be calculated as:
Pt = (1.5 X P X St) / S
………..Where, Pt : minimum test pressure.
P : internal design pressure.
St: allowable stress at test temperature.
S : allowable stress as design temperature.
C. If a test pressure as per above would produce a stress in excess of the yield strength at test temp. The test pressure may be reduced to maximum pressure that will not exceed the yield strength at test temp.
D. If the test pressure of piping exceeds the vessel pressure and it is not considered practicable to isolate piping from vessel, the piping and vessel may be tested together at test pressure of the vessel when approved by owner and provided the test pressure for vessel is not less than 115% of piping design pressure adjusted for temperature as per point No.B.
15. How do you calculate the pipe spacing?
Pipe Spacing (mm) = ( Do+ Dt) / 2 + 25mm + Thickness of Insulation (mm).
Where: D0 : OD of small size pipe (mm).
Dt: OD of flange of large size pipe (mm).
16. Which fluid is used in heat exchanger in shell side and tube side?
Generally corrosive fluid is used from the tube side (as tube can be easily replaced) and cleaner fluid is used from shell side. Sometimes hot fluid is also used from the shell side.
17. What is reynolds number and what is the value of reynolds number upto which the flow is laminar?
It’s a dimension less number to classify the nature of flow.
Re= a v d / f
………Where, Re: raynold’s no.
a : density of fluid.
d : diameter of Pipe.
v : average velocity of fluid.
f : viscosity of fluid.
flow is laminar upto Re=2100

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